The present invention relates to a delay fuze for exploding an explosive shell during flight at a predetermined time after launch, and particularly to an electrical delay fuze for a projectile launched from a shoulder-fired gun that is adjustable for air burst at particular ranges up to the instant of firing. For this purpose, part of the fuze is carried by the gun and easily adjustable by the operator to make last-minute corrections of the firing delay after observing previous target misses due to incorrect range settings. A typical target is a prone man who is not precisely located but presents a larger area to an air burst than to a ground burst. For example, the invention may be used with 26 mm ammunition fired from a semi-automatic weapon, with a 2.2 lb. secs. recoil impulse and a maximum range of about 1000 meters. The range of the target may be estimated visually or by a range-finding device. The estimated effectiveness of the new munition concept, as measured by the expected number of incapacitations per combat load, is several times that of either the current M-406 or M-397 40 mm munitions. The time delay component is additional to the normal mechanical impact component and the mandatory "safe-and-arm" component of the projectile fuze.
Conventional air-burst fuzes of the proximity type are too expensive and are unsuitable for the area fire concept which involves short range ground-to-ground combat situations and a relatively flat trajectory of the projectile. The costs of conventional digital time delay fuzes range from $10 to $25, making them impractical for the present concept. Conventional time delay fuzes containing a variable resistance tend to be impractical in a small volume fuze. Variable time delay has also been obtained by precharging a timing capacitor to a variable voltage before the start of the time interval. This has been done in the XM-433 fuze designed for a large caliber round.
In the present invention, the fuze comprises a DC power source, e.g. a battery, and a voltage divider carried by the gun and adapted to successively apply two different DC voltages to a firing circuit in the projectile. This firing circuit comprises two capacitors, initially connected in parallel with each other to be charged by the battery, prior to firing the gun, to two different voltages, switching means for subsequently connecting the two capacitors in series with each other and with a timing resistor, and means for connecting the two capacitors to the cathode, anode and gate of a unijunction transistor in such manner that, after setback and projectile separation from the gun at firing, the two series capacitors discharge through the timing resistor until the anode is sufficiently positive relative to the gate to cause the transistor to discharge through an electrical detonator and explode the projectile. The fuze may also include a mechanical impact fuze adapted to close a switch shunting the timing resistor, which causes the transistor to discharge, in the event of impact prior to the end of the electrical delay period.